feat(ast): model statements, blocks and nested conditionals

Keeps written order, so a later branch can observe the result of a check that
appeared earlier in the file.

lib/ast.ml

@@ -0,0 +1,45 @@
+type value = { text : string; span : Span.t; quoted : bool }
+
+type expr =
+  | Atom of value
+  | Not of expr * Span.t
+  | And of expr * expr
+  | Or of expr * expr
+  | Cmp of value * string * value
+
+type field = { key : string; kspan : Span.t; values : value list }
+
+type fitem =
+  | FField of field
+  | FIf of (expr * fitem list) list * fitem list
+
+type block = {
+  kind : string;
+  kspan : Span.t;
+  bname : string;
+  nspan : Span.t;
+  items : fitem list;
+}
+
+type check =
+  | Header of value
+  | Func of value * value option
+  | Symbol of value * value
+  | Sizeof of value
+  | Compiles of value * value
+
+type stmt =
+  | Project of value
+  | Set of value * value list
+  | Append of value * value list
+  | Option of value * value * value list
+  | Include of value
+  | Subdir of value
+  | If of (expr * stmt list) list * stmt list
+  | Check of check * Span.t
+  | Pkg of value * value option * Span.t
+  | ConfigHeader of value
+  | Message of value * value list
+  | Blk of block
+
+let v text span = { text; span; quoted = false }